The present invention relates to a semiconductor light-emitting device which is formed such as of nitride semiconductor, and to a fabrication method thereof.
In recent years, while green to blue light-emitting diodes (LEDs) formed of nitride semiconductor have been realized, shorter-wavelength LEDs are also under development for application to white LEDs.
Generally, in fabricating such a short-wavelength LED, an underlying layer of gallium nitride (GaN) is laminated on a sapphire substrate, and thereafter, nitride semiconductor crystal is grown on the underlying layer, thereby forming a grown layer having an active layer or the like. In order to form a grown layer having a good crystallinity, it is indispensable to form an underlying layer of GaN on the sapphire substrate.
However, since gallium nitride (GaN) absorbs light of wavelengths shorter than 360 nm, the underlying layer is removed from the grown layer to prevent light produced in the active layer of the grown layer from being absorbed by the underlying layer when the light is transmitted out of the device.
The gallium nitride layer to serve as the underlying layer may be removed by the following suggested techniques. That is, the sapphire substrate and the underlying layer may be removed from the grown layer by mechanical polishing. Alternatively, the sapphire substrate may be removed by laser lift-off, and thereafter, the underlying layer may be removed from the grown layer by etching or mechanical polishing.
On the other hand, commercially available blue LEDs of nitride semiconductor are designed to allow light produced in the active layer to be transmitted through a planar transparent electrode or a translucent electrode, which is provided as a drive current supply p-electrode on a light extraction face (p-type layer) of the grown layer, or alternatively to be transmitted through a planar sapphire substrate.
However, it is difficult to improve light extraction efficiency in the case of the structure in which light is transmitted through the planar transparent electrode or the translucent electrode provided as the p-electrode. This is because the refractive index of nitride semiconductor is about 2.5 and for example, ITO used as the transparent electrode also has a high refractive index, thus causing the majority of light produced in the active layer to be reflected on a planar semiconductor/resin interface.
It is also difficult to improve light extraction efficiency even in the case of the structure in which light is transmitted through the planar sapphire substrate. This is because the majority of light produced in the active layer is reflected on a semiconductor/sapphire/resin interface.
On the other hand, as a suggested technique to improve light extraction efficiency in this manner, GaP-based LEDs emitting red light is provided with a coarse-grained light extraction face by wet etching, so that the light produced in the active layer is reflected with difficulty on the coarse-grained light extraction face, thereby providing an improved light extraction efficiency (Japanese Patent Application Laid-Open No. 2000-299494 and Japanese Patent Application Laid-Open No. Hei 4-354382).
However, it is difficult to properly remove the entire underlying layer (gallium nitride layer) having a large area using the technique for removing the gallium nitride layer employed as the underlying layer from the grown layer only by mechanical polishing, or by laser lift-off and etching or mechanical polishing as mentioned above.
Since the nitride semiconductor is chemically stable and mechanically hard, it is also difficult to provide a coarse-grained light extraction face to the GaP-based LED by wet etching as disclosed in Japanese Patent Application Laid-Open No. 2000-299494 or Japanese Patent Application Laid-Open No. Hei 4-354382. Accordingly, the light extraction efficiency cannot be easily improved.
Additionally, in the case where light is extracted through the sapphire substrate in order to prevent light from being absorbed by the translucent electrode or transparent electrode, the light extraction efficiency cannot be improved by providing a coarse-grained surface because the sapphire substrate is more difficult to process than the nitride semiconductor.
The present invention was developed in view of the aforementioned conventional problems. It is therefore an object of the present invention to provide a fabrication method of a semiconductor light-emitting device having the underlying layer removed efficiently from the grown layer.
It is another object of the present invention to provide a fabrication method of a semiconductor light-emitting device which allows for improving light extraction efficiency.
It is still another object of the present invention to provide a fabrication method of a semiconductor light-emitting device which allows for efficiently removing the underlying layer from the grown layer and improving light extraction efficiency.